Electric resistivity of a downhole formation is often measured from a wireline in a well bore to analyze formation parameters. Induction resistivity tools induce a magnetic field into the formation; and thus, are different from laterolog resistivity systems, where an electric current is passed through the formation.
U.S. Pat. No. 6,677,756 to Fanini et al., which is herein incorporated by reference for all that it contains, discloses an induction tool for formation resistivity evaluations. The induction tool provides electromagnetic transmitters and sensors suitable for transmitting and receiving magnetic fields in radial directions.
U.S. Pat. No. 6,359,438 to Bittar, which is herein incorporated by reference for all that it contains, discloses a resistivity tool for use in a logging while drilling (LWD) system and includes a transmitter array with multiple transmitters positioned above a pair of receivers. The transmitters of the transmitter array are selectively energized, causing current to be induced in a collar of the resistivity tool.
U.S. Pat. No. 6,577,129 to Thompson et al., which is herein incorporated by reference for all that it contains, discloses an electromagnetic wave propagation resistivity borehole logging system comprising multiple groups of electromagnetic transmitter-receiver arrays operating at three frequencies.
U.S. Pat. No. 6,538,447 to Bittar, which is herein incorporated by reference for all that it contains, discloses a multi mode resistivity tool for use in a logging-while-drilling system and includes an asymmetric transmitter design with multiple transmitters capable of generating electromagnetic signals at multiple depths of investigation.
U.S. Pat. No. 7,141,981 to Folbert et al., which is herein incorporated by reference for all that it contains, discloses a resistivity logging tool suitable for downhole use and includes a transmitter, and two spaced apart receivers. The measured resistivities at the two receivers are corrected based on measuring the responses of the receivers to a calibration signal.
U.S. Pat. No. 6,218,842 to Bittar et al., which is herein incorporated by reference for all that it contains, discloses a resistivity tool for use in LWD systems and includes an asymmetric transmitter design with multiple transmitters capable of generating electromagnetic (EM) signals at multiple frequencies.
U.S. Pat. No. 5,045,795 to Gianzero et al., which is herein incorporated by reference for all that it contains, discloses a coil array which is installed on a measurement while drilling (MWD) drill collar for use in a resistivity logging system. The drill collar is provided with upper and lower coil support rings. The support rings are toroids which support individual coil segments, and are connected by suitable magnetic shorting bars. The coil segments and shorting bars inscribe a specified solid angle or azimuthal extent.
U.S. Pat. No. 5,606,260 to Giordano et al., which is herein incorporated by reference for all that it contains, discloses a microdevice that is provided for measuring the electromagnetic characteristics of a medium in a borehole. The microdevice includes at least one emitting or transmitting coil and at least one receiving coil. The microdevice generates an A.C. voltage at the terminals of the transmitting coil and measures a signal at the terminals of the receiving coil. The microdevice also includes an E-shaped electrically insulating, soft magnetic material circuit serving as a support for each of the coils and is positioned adjacent to the medium in the borehole.
U.S. Pat. No. 6,100,696 to Sinclair, which is herein incorporated by reference for all that it contains, discloses a directional induction logging tool that is provided for measurement while drilling. This tool is preferably placed in a side pocket of a drill collar, and comprises transmitter and receiver coils and an electromagnetic reflector.
U.S. Pat. No. 6,163,155 to Bittar et al., which is herein incorporated by reference for all that it contains, discloses a downhole method and apparatus for simultaneously determining the horizontal resistivity, vertical resistivity, and relative dip angle for anisotropic earth formations.
U.S. Pat. No. 6,476,609 to Bittar et al., which is herein incorporated by reference for all that it contains, discloses an antenna configuration in which a transmitter antenna and a receiver antenna are oriented in nonparallel planes such that the vertical resistivity and the relative dip angle are decoupled.
U.S. Pat. No. 6,900,640 to Fanini et al., which is herein incorporated by reference for all that it contains, discloses a tool that provides electromagnetic transmitters and sensors suitable for transmitting and receiving magnetic fields in radial directions that are orthogonal to the tool's longitudinal axis with minimal susceptibility to errors associated with parasitic eddy currents induced in the metal components surrounding the transmitter and receiver coils.